This is a multicenter, randomized, comparative study of two prophylactic regimens (daily dapsone and daily atovaquone) in HIV-infected patients with either a history of PCP or a CD4+ cell count of < 200/mm3 or < 15% of the total lymphocyte count, who are intolerant of trimethoprim and/or sulfonamides. The two regimens will be compared as to toxicity; incidence of the combined endpoint of PCP, extrapulmonary pneumocystosis, and death; incidence of toxoplasmosis; and mortality rates. (ACTG 277)